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Now that Royal Dutch Shell(NYSE:RDS-A) has offered to buy BG Group for $69.6 billion, all attention is moving from a laser focus on this merger to a broader view of the industry and the wave of mergers that many believe are on the horizon.

The reason? Big companies can afford to wait out the slump – a temporary slump, they hope – in oil prices, while companies not so large keep losing money no matter how much oil they produce.

Other mid-sized and small energy companies have done even worse. London-based Tullow Oil's (LSE:TLW) stock has lost around 60 percent of its value and another British-based company, Premier Oil (LSE:PMO), has done nearly as badly, losing about 50 percent of its stock value.

Shell has set a very clear example of the value of its effort to buy BG: eliminating redundant expenses. Why have two energy companies do the same thing, spending twice the money? Certainly it's sometimes difficult for one company to take on the tasks of another, but sometimes it's just a matter of careful paring.

That Shell-BG deal was also sweetened for the sellers: Shell not only offered to pay BG stockholders 50 percent more than their stock was worth, but would also leave them with a 19 percent stake in the new company. That kind of dealing leads to takeovers that are less than hostile and therefore easy to close.

That's one reason why at least one analyst spoke with such certainty of many mergers and acquisitions (M&As) on the horizon. "This could signal the start of a wave of M&A activity," said Jean-Luc Romain, an analyst at CM-CIC Securities in Paris. "Exxon(NASDAQ:XON) has made no secret that it's on the hunt for acquisitions."

The logic is clear, and a prime reason why occurred only last fall, at OPEC's November meeting in Vienna. The cartel was in a merchant's worst possible position: a glut in production and a lull in demand, leading to falling prices. But under Saudi influence, OPEC decided to maintain its production levels at 30 million barrels per day.

Saudi Oil Minister Ali al-Naimi said the goal was to force smaller oil companies -- particularly US shale producers, whose extraction process is expensive -- out of business so OPEC could regain lost market share.

Some OPEC members, notably Venezuela and Iran, complained that they couldn't afford a protracted price war. Other members of the cartel, though, especially Arab states on the Persian Gulf, are rich enough to stay afloat, even if their national budgets may feel a bit of a pinch. That pinch will felt least by Saudi Arabia itself, which is widely reported to have currency reserves of about $750 billion.

This works for companies as well as countries. As noted, mid-sized BG lost 20 percent of its stock value during the price slump, yet Shell lost only 10 percent, in part because it is one of the largest energy companies in the world, and may very well surpass ExxonMobil Corp. as the largest if the BG merger is approved.

Because of its size, Shell probably would survive the current depression in oil prices even without buying BG. But by absorbing it, Shell not only saves money, it also acquires BG's gas resources in East Africa, a large LNG gas project in Australia and the Santos Basin oil fields off the shore of Brazil -- all without spending a nickel on exploration.

Is it any wonder then that Exxon, too, is eager to buy up a smaller competitor or that any other large oil company would try to do the same?